Methods of producing hydrogen iodide electrochemically

Abstract

Improved electrochemical processes for producing high purity grades of hydrogen iodide without developing cell fouling iodine solids through oxidation of iodide at the anode back migrating through ion exchange membrane into anolyte compartment. Two and three compartment electrochemical cells have anolyte solutions with chemical agents for oxidizing back migrating iodides to soluble iodine species to avoid build up of iodine solids on key cell components. Other embodiments include processes with undivided electrochemical cells fitted with hydrogen depolarized anodes, optionally operating electrogeneratively producing at least some of its own power requirements while simultaneously producing HI, or processes of making high purity HI with multi-phase aqueous/non-aqueous anolytes for solubilizing iodine solids as they develop in the anolyte compartment.

We claim: 
     
       1. A method of producing hydrogen iodide electrochemically, which comprises the steps of: (i) providing a compartmentalized electrochemical cell having an anode in an anolyte compartment and a cathode in a catholyte compartment;   (ii) introducing into said catholyte compartment an aqueous electrolyte comprising solubilized iodine;   (iii) introducing into said anolyte compartment an aqueous solution comprising an oxidizing agent capable of oxidizing back migrating iodide ions to a soluble iodine species without the production of cell fouling amounts of iodine;   (iv) impressing a voltage across said anode and cathode to produce at least protons in the anolyte compartment, and hydrogen iodide in the catholyte compartment.   
     
     
       2. The method of claim 1 wherein the aqueous solution of step (iii) comprises an oxidizing agent selected from the group consisting of halogenated oxidizing acid, non-halogenated peracid, hydrogen peroxide, ozone, and mixtures thereof. 
     
     
       3. The method of claim 1 wherein the oxidizing agent of the aqueous solution of step (iii) comprises a halogenated oxidizing acid selected from the group consisting of iodic, periodic, bromic, chloric, perchloric acids and mixtures thereof. 
     
     
       4. The method of claim 1 wherein the oxidizing agent of the aqueous solution of step (iii) comprises a halogenated oxidizing acid selected from the group consisting of iodic acid, periodic acid and mixtures thereof. 
     
     
       5. The method of claim 4 wherein both hydrogen iodide and periodic acid are simultaneously produced as electrolysis products. 
     
     
       6. The method of claim 1 wherein the anolyte and catholyte compartments of the compartmentalized electrochemical cell of step (i) are separated by a porous separator and hydrostatic pressure on said aqueous solution in the anolyte compartment is increased relative to the hydrostatic pressure in the catholyte compartment, or the hydrostatic pressure on said solution in the catholyte compartment is decreased relative to the hydrostatic pressure in the anolyte compartment. 
     
     
       7. The method of claim 1 wherein the anolyte and catholyte compartments of the compartmentalized electrochemical cell of step (i) are separated by an ion-exchange membrane. 
     
     
       8. The method of claim 6 wherein the cathode is a high surface area cathode. 
     
     
       9. The method of claim 8 wherein the high surface area cathode is comprised of carbon. 
     
     
       10. The method of claim 9 wherein the carbon cathode is comprised of a high surface area graphite felt. 
     
     
       11. The method of claim 6 wherein the cathode is a solid graphite plate. 
     
     
       12. The method of claim 6 wherein the anode is a member selected from the group consisting of noble metal-containing anode, dimensionally stable anode, graphite-containing anode, substoichiometric titanium oxide-containing anode and lead oxide-containing anode. 
     
     
       13. The method of claim 7 wherein the ion-exchange membrane is a cation exchange permselective membrane. 
     
     
       14. The method of claim 13 wherein cation exchange permselective membrane is a perfluorosulfonic acid membrane. 
     
     
       15. The method of claim 13 wherein the membrane and anode of the electrochemical cell are in the configuration of a solid polymer electrolyte composite, and the anode comprises a material selected from the group consisting of a noble metal, noble metal oxide and lead oxide. 
     
     
       16. The method of claim 7 wherein the membrane and at least one of the electrodes of said electrochemical cell are formed into a solid polymer electrolyte composite. 
     
     
       17. The method of claim 1 wherein the anode is a hydrogen depolarized anode. 
     
     
       18. The method of claim 1 wherein HI is produced in a continuous or semi-continuous mode. 
     
     
       19. The method of claim 14 wherein the continuous mode is performed by the steps of distilling HI from the catholyte, and condensing the vapor to form a distillate rich in HI. 
     
     
       20. The method of claim 18 wherein the continuous or semi-continuous mode is performed by the steps of removing a solution of HI from the catholyte of the compartmentalized electrochemical cell, and further electrolyzing said removed solution in a secondary polishing cell to convert a substantial portion of the residual solubilized iodine to HI. 
     
     
       21. A method of producing hydrogen iodide electrochemically, which comprises the steps of: (i) providing a three compartment electrochemical cell having an anode in an anolyte compartment, a cathode in a catholyte compartment, and a central compartment disposed between said anolyte and catholyte compartments, said electrochemical cell having a cation exchange membrane separating said anolyte and central compartments and a anion exchange membrane separating said catholyte and central compartments;   (ii) introducing into said catholyte compartment an aqueous solution comprising solubilized iodine;   (iii) introducing into said anolyte compartment an aqueous solution comprising an oxidizing agent capable of oxidizing back migrating iodide ions to a soluble iodine species without the production of cell fouling amounts of iodine;   (iv) introducing into said central compartment an aqueous solution of an electrolyte comprising HI, and   (v) impressing a voltage across said anode and cathode to produce iodide ions at the cathode and protons at the anode, the iodide ions from the catholyte and protons from the anolyte passing through their respective membranes into said central compartment to form high purity hydrogen iodide.   
     
     
       22. The method of claim 21 wherein the aqueous solution in the anolyte compartment of step (iii) comprises an oxidizing agent selected from the group consisting of halogenated oxidizing acid, hydrogen peroxide, ozone, non-halogenated peracid and mixtures thereof. 
     
     
       23. The method of claim 22 wherein the concentration of solubilized iodine in the catholyte compartment is at levels sufficiently low to minimize transport of solubilized iodine to the central compartment. 
     
     
       24. A method of producing hydrogen iodide electrochemically, which comprises the steps of: (i) providing a three compartment electrochemical cell having an anode in an anolyte compartment, a cathode in a catholyte compartment, and a central compartment disposed between said anolyte and catholyte compartments, said anolyte and catholyte compartments being separated from said central compartment by first and second spaced cation exchange membranes;   (ii) introducing into said catholyte compartment an aqueous electrolyte solution comprising solubilized iodine;   (iii) introducing into said anolyte compartment an aqueous solution comprising an oxidizing agent in an amount at least sufficient to chemically oxidize back migrating iodide ions from the central compartment, said oxidizing agent selected from the group consisting of halogenated oxidizing acid, non-halogenated peracid, hydrogen peroxide, ozone, and mixtures thereof;   (iv) introducing into said central compartment an aqueous electrolyte solution comprising HI, and   (v) impressing a voltage across said anode and cathode to produce iodide ions at the cathode and protons at the anode, said protons passing through said first cation exchange membrane into the central compartment and through said second cation exchange membrane into the catholyte compartment to react with the iodide ions therein to form high purity hydrogen iodide without cell fouling amounts iodine being produced.   
     
     
       25. The method of claim 24 wherein the central compartment of the electrochemical cell receives protons from the anolyte compartment and iodide ions from the catholyte compartment as a flush solution of HI, the HI in the central compartment maintained at a lower concentration than the concentration of HI in the catholyte compartment. 
     
     
       26. The method of claim 24 wherein the aqueous solution of the anolyte of step (iii) comprises an acidic electrolyte selected from the group consisting of oxidizing acids and non-oxidizing acids. 
     
     
       27. A method of producing hydrogen iodide electrochemically, which comprises the steps of: (i) providing an undivided electrochemical cell having a cathode and a hydrogen depolarized anode;   (ii) introducing into the electrochemical cell an electrolyte comprising solubilized iodine;   (iii) providing a voltage source for said anode and cathode to produce iodide ions at said cathode, and   (iv) feeding a source of hydrogen to said hydrogen depolarized anode to form protons for reacting with the iodide ions.   
     
     
       28. The method of claim 27 wherein said hydrogen depolarized anode comprises a dry side and a wet side, and HI formed is removed from the electrochemical cell on the wet side of said anode. 
     
     
       29. The method of claim 27 wherein the electrochemical cell is operated electrogeneratively. 
     
     
       30. A method of producing hydrogen iodide electrochemically, which comprises the steps of: (i) providing a membrane divided electrochemical cell having an anode in an anolyte compartment and a cathode in a catholyte compartment;   (ii) introducing into said catholyte compartment an aqueous electrolyte comprising solubilized iodine;   (iii) introducing into said anolyte compartment a liquid comprising an aqueous phase and a non-aqueous phase having an electrochemically stable, iodine solubilizing organic solvent;   (iv) impressing a voltage across said anode and cathode to produce iodide ions at the cathode and protons at the anode, and   (v) forming hydrogen iodide in the catholyte compartment from protons from the anolyte compartment passing through said membrane and reacting with iodide ions without crystalline iodine forming in the anolyte compartment.   
     
     
       31. The method of claim 30 wherein the organic solvent of the non-aqueous phase in the anolyte compartment is a halogenated organic solvent. 
     
     
       32. The method of claim 31 including the steps of withdrawing at least a portion of the aqueous and nonaqueous phases from the anolyte compartment; allowing said aqueous phase and non-aqueous phase to form separate layers, recovering crystalline iodine from said non-aqueous phase and returning the iodine depleted non-aqueous phase and aqueous phase liquid to the anolyte compartment. 
     
     
       33. The method of claim 30 wherein the membrane of said electrochemical cell of step (i) is a perfluorosulfonic acid type cation exchange membrane. 
     
     
       34. The method of claim 33 wherein the anode and cation exchange membrane comprise a solid polymer electrolyte composite. 
     
     
       35. The method of claim 31 wherein said organic solvent is a member selected from the group consisting of methylene dichloride, ethylene dichloride and trichloroethylene.